Direct current ammeter and voltmeter



Feb. 10,1942. A. J. CORSON Erm.

DIRECT CURRENT AMMETER AND VOLTMETER F iled April-'7, 1959 mmunmmmum 1 nvencors Almon' J. Corson, Ralph M. Rowell,

The]? Attorney.

Patented Feb. 10, 1942 DIRECT CURRENT AMJVIETER AND .VOLTMETER Almon J. Corson, Swampscott, and Ralph M. Rowell, Lynn, Mass., assignors to General Electric Company, a corporation of New York Application April 7, 1939, ,Serial No. 266,626

3 Claims.

Our invention relates to electrical instruments and concerns particularly current responsive devices adapted for use in direct current circuits.

It is an object of our invention to provide an improved, highly sensitive, light, compact, sturdy, current responsive instrument which has a long scale as -great, or greater, than 250 degrees in angular length, which is highly resistant to changes in calibration and which is shielde against external fields.

Other and further objects and advantages will become apparent as the description proceeds.

In carrying out our invention in its preferred form, we provide an annular permanent magnet having a peripheral length of substantially 260 degrees composed of high coercive force magnetic material magnetized radially so that poles of opposite polarity are formed at the outer and inner cylindrical surfaces of the magnet and we provide an inner annular member or pole piece which is'mounted within the magnet and spaced therefrom to form an annular air gap. We also. provide a current conducting coil which links the annular pole piece, has a side passing through said pole piece substantially along the axis of said pole piece and said magnet, is rotatable about said side and has another side adapted to sweep through the annular air gap. Suitable means are, of course, provided for forming a magnetic -return circuit between the annular inner pole piece and the outer surface of the annular magnet. An indicating pointer may be connected to the rotatable coil in accordance with the customary practice, and in this manner an indicating instrument is" formed having a. scale range as great or greater than .250 degrees in angular length. v

Our invention may be understood more readily from the following detailed description when considered -in connection with the accompanying drawing and those features of the invention which are believed to be novel and patentable will be pointed out in the claims appended here- -to. In the drawing Figure 1 is a schematic diagram of apparatus which may be utilized for magnetizing the permanent magnets used in our current responsive instruments. Figure 2 is an elevation of an instrument unit or torque-producing unit for a current responsive indicating instrument forming one embodiment of our invention, certain portions of the remaining structure of the complete instrument being shown fragmentarily or in section. Figure 3 is an elevational view of the apparatus of Figure 2 together with a magnetic keeper which may be utilized for preserving the strength of the permanent magnet during assembly of the apparatus, or

when for any reason the apparatus is disassembled. Figure 4 is an elevational view corresponding to Figure 2 but showing additional details of construction of the supporting frame and other parts of a complete indicating in-' strument. Figure 5 is a horizontal transverse section of the magnetic flux-carrying members shown in Figure 3 as cut, by the plane 5-5, and Figure 6 is an exploded diagrammatic perspective view of a portion of the apparatus of Figure .1, including a portion of an instrument being magnetized. Like reference characters are utilized throughout the drawing to designate like parts. 7

Referring now more in detail to the drawing, we have shown in Figure 2 a torque producing element of an indicating'instrument, such as a voltmeter or an ammeter. There is a radially magnetized annular permanent magnet II cooperatingwith an annular pole piece I2, mounted within the magnet II, and a current conducting coil I3 eccentrically mounted to rotate about one of its sides while linking the annular pole piece I2 and having another side I5 sweeping through an air gap I 6 formed between the permanent magnet II and the inner annular pole piece I2.

It will be understood that the outer cylindrical surface of the annular pole piece I2 is spaced sufiicientl'y from the inner surface of the pole piece of the magnet I I to provide an annular air gap of sufficient width for the side I5 of the coil I3.

For the dual purpose of shielding the instrument and forming a return magnetic path, a member 11 is provided which may be substantially ring-shaped having a surface abutting the outer cylindrical surface IB of the magnet II I and having ends I9 brought into magnetic connection with the inner annular pole piece I2. To facilitate providing a compact structure the annular magnet II may have a segment thereof, preferably not more than degrees in angular length, cut out to permit flux-carrying members 20 to extend radially from the inner annular pole piece I2 to the shielding or ring member I1, and for the sake of convenience in assembly, the ring member I! may also have a segment at the bottom out out so that the fluxcarrying members 20 are fitted against transverse surfaces at the ends I9 of the ring member I1. The magnet II may thus be referred to as a segmental annulus. For the. sake of greater efliciency and greater ease in obtaining accuracy of construction in the manufacturing process, the annular pole piece l2 and the flux-carrying members 20 are preferably made integral to form a U-shaped core member designated by the reference numeral 2!, and consisting of an annular yoke portion l2 and leg portions 20. For the sake of facilitating the provision of uniform pole strength along the inner cylindrical surface of the permanent magnet ll, an annular pole piece 22 is preferably provided having an inner cylindrical surface 23 uniformly spaced from the outer cylindrical surface of the annular pole piece I2.

The permanent magnet Ii is preferably composed of a material having a relatively high co ercive force such as either cast or sintered material composed, for example, of an alloy of iron, nickel, aluminum and cobalt, such as described in United States Patents 1,947,274 and 1,968,569 to Ruder. Various modifications in specific percentages of the several constituents may be employed, for example:

' Percent Nickel 30 Aluminum 12 Iron, 375% silicon and the balance impurities providing a coercive force of 700 oersteds 5'7 Nickel 17 Aluminum Cobalt 12 Copper 6 Iron, providing a coercive force of 600 oersteds- 55 and with other slight variations in composition various coercive forces exceeding 600 oersteds may be obtained. Suitable means are preferably employed for uniting the permanent magnet II with the pole piece 22 and the ring member H to form a unitary magnetic field member.

If a cast magnet is to be used the unitary field member is produced by die-casting the magnet ll into a form or mold in which the pole piece 22 and the ring member l1 have been inserted. In the case of sintered magnets, the sintered material is pressed into a mold having the pole piece 22 and the ring member I1 as inserts and is thereupon subjected to suitable heat and pressure.

The pole piece 22 is composed of any suitable magnetic material which is relatively permeable. Such a material as an alloy of m /2% nickel with the remainder iron may be utilized but satisfactory results may also be obtained by utilizing cold rolled steel.

The ring member I! and the U-shaped core member 2| are likewise preferably composed of magnetic material having relatively high permeability, such as the nickel-iron alloy mentioned, or soft steel. For the sake of convenience in manufacture and accuracy of surface contour and alignment the members I! and 2| may be made up of punched steel laminations stacked together and secured in any suitable manner, as by means of nonmagnetic hollow rivets 24 composed of' brass, for example, and a rivet 25 at the yoke portion of the core member 2|, which need not be a hollow rivet. It will be understood that the hollow rivets "are utilized for the sake of convenience in mounting on the supporting structure by bolts or screws.

For supporting the eccentrically rotatable coil l3, a shaft 26.,may be provided which is coaxial with the annular magnet l I and the-annular insupporting the movable elements. Such a frame is showmfragmentarily at 29. Inasmuch as our invention does not reside in the specific details of the arrangement for providing structural support, the frame 29 need not be described or iilustrated in detail. It may be'similar to that shown in the copending application of one of the joint applicants Almon J. Corson, Serial No. 236,891, filed October 25, 1938, now Patent No. 2,218,376, in which the frame is E-shaped having relatively long upwardly extending arms 21 and 28 at each end of a horizontally extending base 38 and having intermediate of the base 30 an upwardly extending bracket 3| (Figure 4) to which the ring member l1 and the core member 2| may be secured by suitable bolts or screws 32. It will be understood that a suitable pro,- tective case may be provided and that a suitable indicating scale may be mounted in the case.

It the apparatus is to be in the form of an indicating instrument, it will be understood that a suitable pointer 33 (fragmentarily shown in Figure 4) may be provided for cooperating with the scale. As will be understood by those skilled in the art, the pointer 33 is carried by the shaft 26 and, if desired, counterweight carrying arms 34 may also be secured to the shaft 26 for the though we have described our invention in con nection with an embodiment in the form of an indicating measuring instrument, it will be understood that our invention is not limited thereto and obviously includes various types of current responsive instruments such as contactmaking voltmeters and ammeters, relays, telemeters and recorders.

Inasmuch as high coercive force magnetic material, such as that of which the magnet II is preferably composed is relatively sensitive to a great increase of the external magnetic reluctance, I preferably provide a keeper 25 of permeable magnetic material for shunting the external magnetic circuit of the magnet ll before the core 2| is inserted, or whenever for any reason the apparatus is disassembled. It will be understood, of course, that the keeper 3! is v applied immediately upon the magnetization of the permanent magnet ll before any demagnetizing field appears and that the keeper 3! is removed only after thecore 2| hasbeen inserted to provide the flux path for the magnet II in order to prevent weakening the latter. In order to facilitate the use of the magnetic keeper 2! the shielding ring l1 and the annular pole piece 22 are caused to project slightly in the axial direction beyond the magnet II, as shown at 33 in Figure 5, to provide contact surfaces meeting the surface of the keeper 2!. It will be under- .stood, of course, that the heads of the rivets 24 fastening the laminations of the shielding ring I! together are properly countersimk to 'in Figure 2, the contour departs from a circular leave a flat surface for contacting the keeper 35. As shown in Figure 3 the keeper 35 is substantially in the shape of a flat washer with a segment thereof cut out to correspond to the shape of the permanent magnet II, and the opening or the gap in the keeper 35 is made large enough to facilitate sliding the keeper on to the magnet structure before the latter is removed from the magnetizer, as will be explained in connection with Figures -1 and 6.

It will be apparent that our construction lends itself readily to easy, rapid, and accurate assembly of current responsive instruments. The shaft 26, carrying the current conducting coil 3 and the pointer 33, is first mounted within the bearings carried by the frame arms 21 and 23. The coil I3 is turned to the horizontal position as shown in Figure 2 and the core 2| is then linked with the coil by passing one of the legs through the coil l3, whereupon the core 2| is bolted to the bracket 3| by bolts such as the bolts 32 (Fig. 4). After the permanent magnet II has been magnetized, while the keeper is' still in place, the unitary permanent magnet structure consisting of the magnet II, the pole piece 22 and the shielding ring I1 is passed axially toward the core 2| and the coil l3 so as to surround them and is also secured to the frame bracket 3| utilizing the bolts 32 which pass through the frame bracket 3|. After the torque-producing unit which is shown in Figure 3 has been assembled the keeper 3.5 may be removed by drawing it away in an axial direction.

For the purpose of magnetizing the permanent magnet M an arrangement such as that shown inFigure 1 may be employed. This consists of a magnetic yoke 31 of any desired shape provided with one or more exciting windings. or

cross section.

As will be well understood by those skilled in the art it will be necessary to subject the permanent magnet II to a magnetizing field much greater than that which-it ise ted to maintain in order to magnetize it to its practical limit. After the permanent magnet II has been magnetized and the current passing through the solenoids 33 has been cut off, the high permeability magnetic circuit of the magnetizer consisting of the yoke 31 and the pole pieces 39, 42

and 43, of course, provides a low reluctance magnetic return path which prevents subjecting the permanent magnet I to the .dem'agnetizing force.

' Thereafter, the removable pole pieces 42 and 43 may be slipped off by passing them in axial di-' rections. The removable permanent magnet and keeper may then be brought into place in the torque-producing unit of Figure3 as previously explained.

solenoids 38 suitably placed thereon and having a pair of pole pieces 39 with confronting parallel surfaces 40 and 4|. course, that the yoke 31 and the pole pieces 39 are preferably made of permeable magnetic material and that the solenoids 38 consist of numerous turns of current conducting wire adapted to be connected to a current source, not shown, in order that the extremely high magnetizing force required for magnetizing the high 00- ercive force permanent magnet material may be provided. In order to pass the magnetizing flux from the 'pole pieces 33 radially through the permanent magnet H and in order to facilitate easy insertion of the magnet unit in the magnetizer of Figure 1, a pair of auxiliary movable pole pieces 42 and .4315 provided, one of which, the movable pole piece 42, is substantially in the shape of a truncated cone with a cylindrical continuation. The pole piece 42 has a flat surface 44 adapted to abut the pole face 40 of the magnetizer and has a cylindrical portion 45 of the proper size and shape to provide a slip-fit in the inner surface 23 of the pole piece 22 forming the inside pole face of the permanent magnet ll.

surface 46 so shaped as to form a slip-fit around the shielding ring l1 and having a plane surface 41 adapted to fit against the pole face 4| of, the magnetizer. It will be understood that in Figure 6 in which the parts are shown schematically in reduced size, the shielding ring I! and the removable pole piece :43 have apparently been represented with. substantially cylindrical cooperating surfaces although actually, as shown The removable pole piece 43 is in the approximate shape of a ring, having an inner It will be understood, of

In accordance with the provisions of the patent statutes, we have described the principle of operation of our invention together with the apparatus which we now consider to respect the best embodiment thereof but we desire to have it understood that the apparatus shown is only illustrative and that the invention may be carried out by other means.

What we claim as new and desire to secure by Letters Patent of the United States is:

l. A field structure for a current responsive instrument comprising an annular permanent magnet having a peripheral angular length sub-- stantially as great as 250 degrees composed of high coercive force material magnetized radially, an annular pole piece composed of permeable magnetic material at the inner cylindrical surface of said annular permanent magnet, a shielding member of permeable magnetic material substantially surrounding the outside periphery of said annular permanent magnet, and a U-shaped core member composed of permeable magnetic material having an annular yoke portion surrounded by said annular pole piece and spaced therefrom to form an annular air gap and having leg portions fitted against said shielding member to form with the latter a magnetic return circuit for said annular permanent magnet, saidfleld structure being thereby adapted to cooperate with a current conducting coil eccentrically mounted and rotatable about a side thereof and to receive said coil side within the annular portion of said core member and to have another coil side sweep through said air gap whereby th coil may link the annular portion of said core member.

2. A field structure for a current responsive instrument comprising an annular permanent magnet having a peripheral angular length exceeding degrees composed of high coercive 'nent magnet forming a pole of one polarity, an

annular member of permeable magnetic material mounted coaxially within said annular permanent magnet to form an inner pole piece of opposite polarity, and means composed of magnetically permeable material for providing a magnetic return circuit between the outer cylindrical surface of said permanent magnet and said inner annular pole piece, whereby said field structure is adapted to cooperate with a current conducting coil and to receive one side thereof within said inner annular pole piece mounted to be rotatable about said side and to have another side of the coil sweep through the air gap between said inner annular pole piece and said pole of said annular permanent magnet. 3. A field structure for a current responsive instrument comprising a segmental annular permanent magnet having a peripheral angular length exceeding 90 degrees magnetized radially, an annular pole piece composed of permeable magnetic materia. at th inner cylindrical surface of said annular permanent magnet, an annular member of permeable magnetic material mounted coaxially within said first-mentioned pole piece and spaced therefrom to .Iorm an innor pole piece of opposite polarity separated from the first-mentioned pole piece by an annular air gap, a magnetic return member composed of permeable magnetic material joining the outer cylindrical surface 0! said annular permanent magnet and the said inner pole piece whereby said field structure is adapted to cooperate with a current-conducting coil and to receive one side thereof within said inner annular pole piece mounted to be rotatable about said side and to have another side of the coil sweep through the air gap between said inner annular pole piece and the first-mentioned outer annular pole piece.

- ALMON J. CORSON.

RALPH M. HOWELL. 

